Non-volatile semiconductor memory device for selectively re-checking word lines

ABSTRACT

A method for settling threshold voltages of word lines on a predetermined level in an erasing processing of a non-volatile semiconductor memory device so as to speed up the erasing processing. A word latch circuit is provided for each word line and the threshold voltage of each memory cell is managed for each word line in a selected memory block. Each word latch circuit is shared by a plurality of word lines so as to reduce the required chip area. A rewriting voltage is set for each finished non-volatile memory and the voltage information is stored in the boot area of the non-volatile memory, so that the voltage is recognized by the system each time the system is powered.

FIELD OF TH INVENTION

[0001] The present invention relates to an inflicting voltagecontrolling method employed for the erasing processing for anon-volatile semiconductor memory device and a non-volatilesemiconductor memory device that employs the method.

BACKGROUND OF THE INVENTION

[0002] A floating gate electrode type non-volatile memory element(hereinafter “cell”) stores information according to a different cellcharacteristic decided by the number of electrons accumulated in theirfloating gate electrodes, i.e. a difference between threshold voltagevalues of the cells. Hereinafter, a processing for increasing athreshold voltage is defined as “writing” and a processing fordecreasing the threshold voltage to a predetermined value is defined as“erasing”. Accordingly, the processing for increasing the thresholdvoltage is defined as “erasing” and the processing for decreasing thethreshold voltage to a predetermined value is defined as “write”.

[0003]FIG. 2 shows a graph for denoting distribution of thresholdvoltages of the cells of a general non-volatile semiconductor memorydevice in which each memory array is composed of a plurality of cells. Awriting level is decided by the writing lower limit value V_(pmin) andthe threshold voltages are distributed as denoted by A. In an erasingprocessing, a predetermined voltage is inflicted to the cell group setin the state of distribution A so as to lower each respective thresholdvoltage. Usually, because the erasing characteristic is varied, when aprocessing for lowering a threshold voltage is executed simply as such,the threshold voltages are distributed in a wide-base shape as denotedby B. A cell group (dark portion) of which the threshold voltage isunder the erasing lower limit value V_(emin) (over erasing level)becomes a source of leak current, thereby information cannot be readtherefore correctly. In order to prevent such a problem, therefore, thedata in each cell on the over erasing level is rewritten so that theerasing level is adjusted to a predetermined erasing level distributionC (within a range between the erasing lower limit value V_(emin) and theerasing upper limit value V_(emax)).

[0004] Conventionally, there have been two methods for loweringthreshold voltages: one is a method for managing a plurality of wordlines collectively (hereinafter, to be described as the “blockprocessing”) and the other is a method for managing the threshold ofeach cell in units of a word line (hereinafter, to be described as the“sector processing”). In the case of the block processing, a targetblock including a plurality of word lines is selected according to theinformation stored in the corresponding register. For example, a wellpotential is used as a block selecting signal and a negative voltage isinflicted to all the word lines in the selected block. The chargeaccumulated in a cell is discharged with an electric field generated bythe negative voltage and a positive potential inflicted to the well.Then, the threshold voltage of each cell is checked. When there is acell of which threshold voltage is not under the reference value, anegative voltage is inflicted again to all the word lines in the block.These processings are repeated. When the threshold voltages of all thecells in the block become lower than the reference value, the systemgoes to the next block so as to execute another erasing processing.Because the block processing can process many cells at a time, it takesless time for lowering each threshold voltage.

[0005] The sector processing manages the threshold voltage of each cellin units of a word line. After a negative voltage is inflicted to a wordline, the threshold voltage of the cell connected to the word line ischecked. When a cell of which threshold voltage is not under thereference value is detected, a negative voltage is inflicted again tothe word line. When the threshold voltage of every cell connected to theword line goes under the reference value, the system goes to the nextword line. In the case of this method, a negative voltage for loweringeach threshold voltage is inflicted to just the number of cellsconnected to a word line. Statically, the fewer the word lines, thenarrower the threshold voltage distribution after a processing can bethan that of the block erasing processing. On the other hand, whencompared with the block processing, the processing time becomes longerunavoidably.

[0006] Furthermore, after an erasing processing executed for a cell,another processing is done so as to increase the threshold voltage ofthe cell, which has been decreased excessively. This processing isreferred to as “rewriting”. In any of the block processing and thesector processing described above, it cannot be avoided that some cellsgo into such the over erased state after an erasing processing. This iswhy such the rewriting is done as “a weak writing” for the cells thatare in the over erased state. Because the writing characteristic isvaried just like the erasing characteristic, the threshold voltages mustbe controlled precisely so that they are settled on a predeterminederasing level shown in FIG. 2.

[0007] There is a well-known method for inflicting a voltage to cells soas to write data therein accurately. According to the method, a pulsevoltage inflicted to both drain and gate of each cell is increased byincrements through the hot electron injection method. The thresholdvoltages of cells in which data can be written fast can be controlledeasily and settled on a predetermined erasing level at the initialinfliction of a low voltage. On the other hand, the threshold voltagesof cells in which data is written slowly at a low voltage are controlledso as to be settled quickly on the predetermined erasing level by a highvoltage inflicted in the latter half. Such the writing voltageinflicting method is disclosed in JP-A-Nos. 96591/1996 and 228784/1998.

[0008] In the case of the block processing, since there are many targetcells, the threshold voltages are distributed widely and the number ofcells to be set in the over erased state increases after eachprocessing, suffering from the increase in rewriting processing time. Inthe case of the sector processing, a processing for inflicting a voltageused to lower a threshold voltage must be executed for each of wordlines, causing the voltage inflicting time to increase. In order toreduce the whole erasing processing time, it is required to reduce bothprocessing times for lowering each threshold voltage and rewriting data.In principle, however, it has been difficult for any of the conventionalblock and sector processings to reduce both of such the processingtimes.

[0009] Under such circumstances, it is an object to provide a methodthat enables rewriting to be done so that threshold voltages of memorycells are settled easily on a predetermined level. In order to achievethe above object, the writing pulse voltage is increased by incrementsfor rewriting. And, a minimum level a maximum value, an increment value,and a pulse time should be set properly for each voltage to beinflicted. When there is an excessively high stress recognized on thepulse voltage value, (for example, when the start voltage is excessivelyhigh), in which cells data can be rewritten fast might exceed the upperlimit value of the erasing level. When the start voltage is excessivelylow or when the maximum inflicting voltage is excessively low, therewriting is done slowly which causes the processing time to increase.Setting pulses of the voltage to be inflicted is also an important itemto solve the above conventional problems.

SUMMARY OF THE INVENTION

[0010] Word lines in a selected block, when their threshold voltages arewithin a proper range in an erasing processing, are excluded from anadditional erasing processing respectively. Whether to select a wordline for such an additional erasing processing is done by controlling alatch circuit connected to the word line according to the information ofprocessing end stored in a register. The subject memory is preventedfrom an increase of the occupied area by enabling the latch circuit tobe shared by other word lines.

[0011] When in a rewriting processing, a voltage is inflicted so as toincrease by increments. And, an initial value, a final value, anincrement value, and a pulse width can be set for this voltage. Each ofthose values is selected for each finished non-volatile memory device inaccordance with the characteristic thereof. The selected conditions arestored in the memory chip as non-volatile information and read from thechip and used each time the system is powered.

[0012] Other and further objects, features and advantages of theinvention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] A preferred embodiment of the present invention illustrated inthe accompanying drawings in which:

[0014]FIG. 1 is a block diagram of a non-volatile memory of the presentinvention;

[0015]FIG. 2 is a graph denoting the distribution of threshold voltageson the “writing” level and on the “erasing” level of the non-volatilememory in FIG. 1 respectively;

[0016]FIG. 3 is a block diagram of a non-volatile semiconductor memorydevice in FIG. 1;

[0017]FIG. 4 is a circuit diagram denoting a relationship betweenvoltages inflicted so as to erase data from the non-volatile memory;

[0018]FIG. 5 is a flowchart of the processing of lowering a thresholdvoltage in a cell of the non-volatile memory in FIG. 1;

[0019]FIG. 6 is a flowchart of the processing of deciding a writingvoltage and a rewriting voltage of the present invention;

[0020]FIGS. 7A through 7D and 7F are patterns of voltage pulsesinflicted for rewriting, and FIG. 7E is one example of the contents-of apulse controlled voltage register;

[0021]FIG. 8 is a block diagram of a semiconductor IC system providedwith a non-volatile memory, which is mounted together with the IC systemon the same board; and

[0022]FIG. 9 is a block diagram of a semiconductor IC system providedwith a non-volatile memory, which is mounted together with the IC systemin the same package.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Hereunder, the preferred embodiment of the present invention willbe described with reference to the accompanying drawings.

[0024]FIG. 3 shows a block diagram of a non-volatile semiconductormemory device (memory module MM). A memory array MARY comprises memorycells disposed like a matrix. The threshold voltage of each memory cellcan be raised/lowered electrically. The memory module MM includes a rowaddress buffer XADB, a row address decoder XDCR, a data latch circuitDL, a sense amplifier circuit SA, column gate array circuits YG-Gate,YW-Gate, YT-Gate, a column address buffer YADB, a column address decoderYDCR, a block selecting control circuit BSLC, an input buffer circuitDIB, an output buffer circuit DOB, a multiplexer circuit MP, a modecontrol circuit MC, a control signal buffer circuit CSB, and a built-inpower supply circuit VS, etc.

[0025] The control signal buffer circuit CSB receives at least one ofthe chip enable signal CEb, the output enable signal OEb, the writeenable signal WEb, and the serial clock signal SC, etc so as to outputtiming signals used for triggering internal control signals. The modecontrol circuit MC outputs ready/busy signals to the external terminalR/Bb. The built-in power supply circuit VS receives a supply voltage Vccfrom an external source then outputs voltages required for writing,erasing, and reading. The row/column address buffer XADB/YADB receives arow/column address signal AX/AY from an external terminal so as togenerate and output an address signal to the row/column address decoderXDCR/YDCR.

[0026]FIG. 1 shows a block diagram denoting the word line latchingmethod of the present invention. The memory array MARY has wells WELL01to WELLn separated electrically from each other. A well is a dividedarea provided with a plurality of word lines W01 to Wm. The word linesW01 to Wm are driven by word drivers WD01 to WDm. Each well is selectedby a block selecting signal BSL issued from the block selecting controlcircuit BSLC.

[0027] The row address decoder XDCR includes an erasure unit selectingdecoder DECX1, a word line selecting decoder DDECX2, and latches LTC01to LTC32. In this embodiment, an erasing processing is done for 32 wordlines at a time, so that the erasure unit selecting decoder DECX1selects 32 word lines at a time while the word line selecting decoderDECX2 selects each of those 32 word lines. In addition, registers RES01to RES32 are provided in the controller or CPU for controlling theerasure of data from the memory array corresponding to the latches LTC1to LTC32.

[0028] A cell group in an area decided by the well WELL01 forms a block.In this embodiment, word lines W01 to W32 are included in the firsterasure unit. The block selecting signal BSL decides whether to selecteach block depending on whether the signal takes a positive potential or0V. FIG. 4 shows how a voltage is inflicted to a memory block so as toerase data therein. An electric field formed by a negative voltage(−11V) inflicted to subject word lines and a positive voltage (10V)inflicted to a well is used to discharge the accumulated charge from acell to the board.

[0029] The circuit configuration of the present invention ischaracterized in that the latch circuits LTC01 to LTC32 are shared byword lines other than the word lines W01 to W32. This means that a latchcircuit is shared by a plurality of word lines included in differenterasure units. In FIG. 1, the latch LTC1 is shared by a word line W1 anda word line W33 included in different erasure units. An increase of thechip area caused by a latch circuit LTC is a direct product of the areaper latch circuit and the number of latch circuits. In this embodiment,because a latch circuit is shared by a plurality of erasure units, thechip area of the subject non-volatile memory is not increased due to theemployment of latch circuits.

[0030] Although an erasure unit consists of 32 word lines in FIG. 1, thenumber of word lines is not limited only to 32 per an erasure unit.Although one memory block includes a plurality of erasure units in theabove embodiment, one block may include only one erasure unit. In thiscase, the configuration effectively eliminates the disturbance caused byinfliction of a positive voltage to a well in erasing data therefrom. Onthe other hand, this configuration suppresses the increase of the memoryarray area, which is caused by dividing the well.

[0031] Next, how to erase data from a block will be described withreference to the flowchart shown in FIG. 5. At first, the registersRES01 to 32 are all cleared to “0” (S51). Then, the positive potentialof a block selecting signal, such as 10V, is supplied via the BSL to thewell WELL01 of the block that includes the erasing-targeted word lines.After that, the same negative voltage pulse is inflicted to all the 32word lines included in the subject erasure unit (S52). After thatvoltage pulse infliction, the threshold voltage of each subject cell ischecked sequentially, starting from the word line W01 (S53). When thereis a cell of which threshold voltage does not reach the reference value,the system goes to the next word line for another check (S54). When thethreshold voltage of a cell has reached the reference value, the systemgoes to the next memory cell connected to the same word line (S55) andcontinues the check. When the threshold voltages of all the cellsconnected to the same word line have reached the reference value, thesystem stores the information of processing end “1” in the correspondingregister (S56). After the check is completed for all the 32 word linesincluded in the subject erasure unit, an additional erasing processingis executed for the word line for which “0” is stored in thecorresponding register. When “1” is set for all the values stored in theregisters RES01 to RES32, the system executes the same erasingprocessings for the next erasure unit (S57).

[0032] The system then controls the latches LTC01 to LTC32 according tothe information stored in the register RES so as to inflict −10V pulsevoltage to the word lines which need additional processing and toinflict 0V to the word lines which doesn't need any additionalprocessing, or to set those 32 word lines in the floating potentialstate, respectively. After that, the system checks the threshold voltageof the cell with respect to each word line to which an additionalprocessing has been done. If the processing is not completed yet for anyword line, the system stores “0” in the register corresponding to theword line address again. When the processing is completed for the wordline, the system replaces “0” with “1” in the register. Repeating theseprocessings, it is possible to reduce the number of word lines thatrequire the additional erasing processing by increments, therebyreducing the check time more than the conventional block processing. Inaddition, because a voltage for reducing a threshold voltage isinflicted to only one word line in this embodiment, the thresholdvoltage distribution after a processing becomes equal to that of thesector processing. Consequently, it is possible to reduce the number ofcells reaching the over erasing level that requires rewriting, up tothat of the sector processing, thereby the rewriting time can bereduced.

[0033]FIG. 6 shows a flowchart for executing a rewriting voltageselecting method of the present invention. According to this method, aplurality of cells used for a voltage setting test are selected from afinished non-volatile memory beforehand (S61). Then, the minimum writingvoltage to be estimated from the variation of the manufacturingcharacteristic is decided as the initial value of a voltage used for thetest writing. At first, the initial value is written in all the cells tobe tested (S62). Then, the threshold voltages are read from those cells(S63). Hereinafter, Case A and Case B are used as examples for choosingan inflicting voltage corresponding to the read threshold voltage.

[0034] In Case A, when the center value of the detected thresholdvoltages is under the reference value, e.g., the writing lower limit,the threshold voltages of the test cells are lowered once (S64). Then,the writing voltage is increased by ΔV from the initial voltage (S65),then the writing test is restarted so as to check the thresholdvoltages. Repeating these processings, the final inflicted voltage isdecided as a voltage for the subject chip when the center value of thethreshold voltages of the cells reaches the reference value (S66) afterthe writing. The rewriting voltage is selected as a value correspondingto the writing voltage selected here and decided for the subject chip(S67).

[0035] Unlike the Case A, no processing is repeated in Case B. A voltageto be inflicted to a chip is decided according to the information in atable of inflicting voltages verse the corresponding threshold voltagesafter the writing, the writing voltage, and the rewriting voltage,generated with the measured data in the writing test (S68). In any ofthe Cases A and B, the proper writing voltage and the proper rewritingvoltage are decided for a chip according to the characteristic of eachcompleted non-volatile memory.

[0036] While a rewriting voltage is decided for a chip according to thewriting characteristic of the test cells, the rewriting voltage may bedecided directly by evaluating the rewriting characteristic. Therewriting voltage may also be decided for each chip by varying thestepping voltage that increases by increments. In addition, the presentinvention applies the approach for the “writing” defined above to the“erasing”, i.e. substituting the processing for increasing a thresholdvoltage in “erasing” with a processing for decreasing the thresholdvoltage in “writing”.

[0037] Next, a method for reproducing proper voltage pulses selected inFIG. 6 with reference to FIG. 7 is described. A plurality of voltagepulse patterns are prepared beforehand. The pulse patterns are, forexample, pulses with voltages different by an increment value of ΔV,i.e. pulses whose pulse widths are different from each other by a AV.For example, a plurality of proper voltage pulses look like those shownin FIG. 7(f). In this example, a pattern is selected from the 64predetermined types, and the initial voltage, the increment ΔV, thepulse width, the final target voltage, etc. are specified for the pulsepattern so as to reproduce the same sets of voltage inflictingconditions corresponding to the characteristic of each finished chiptype. FIGS. 7(a) to 7(d) show the four types of sample pulse patterns.FIG. 7(a) shows a pulse voltage having a pulse width T0001. The voltagepulse shown in FIG. 7(a) is selected as the initial pulse of a rewritingvoltage, and VG0001 is specified as the initial value of the inflictingvoltage. The voltage increment value is ΔV. The voltage to be inflictedFIG. 7(a) begins with the pulse having a pulse width T011 at theinflicting voltage of VG00110. The voltage increment value is ΔV. Thevoltage to be inflicted FIG. 7(b) begins with the pulse having a pulsewidth T011 at the inflicting voltage VG01001. The voltage incrementvalue of the voltage in FIG. 7(c) is zero. The voltage is inflicted onlyby a specified number of times. The voltage to be inflicted according toFIG. 7(c) begins with the pulse having a unit time T110 as shown in FIG.7(d) at the inflicting voltage VG01110. After that, the pulse shown inFIG. 7(d) is used until the rewriting is completed.

[0038] Each pre-prepared voltage pulse pattern as described above isstored in a pulse-controlled voltage register as specified informationas shown in FIG. 7(e). In the example shown in FIG. 7(e), 5 bits areassigned to a voltage value and 3 bits are assigned for a pulse width.There are a total of 64 pulse patterns prepared. The specified fourpulse patterns shown in FIG. 7(a) to FIG. 7(d) are stored in theregisters R02, R04, R21, and R63 selected from the registers R01 to R64.The addresses of those four registers for storing specified pulseconditions are stored in the respective portions of the correspondingnon-volatile memory array MM. The area used to store such information isan boot area from which the information is read whenever the system ispowered. An operation voltage selected for each chip is selected fromthe voltage specification information stored in the correspondingregister according to the register address read from the boot area whenthe system is powered.

[0039] Because a voltage inflicted for rewriting is selected accordingto the characteristics of each finished non-volatile memory, the erasingprocessing is done properly. For example, assuming that a non-volatilememory is finished and the average size of the finished memory rangesbetween 0.28 μm to 0.32 μm even when each cell is designed with a gatelength of 0.3 μm. Consequently, a difference of 1V is often generatedamong inflicting voltages to support the same rewriting speed. In such acase, however, it is possible to select a proper inflicting voltage foreach completed memory, so that the memory is assured to operateproperly. In other words, when the information of a proper voltage pulseis stored in the finished non-volatile memory, the specified informationis acknowledged by the system each time the system is powered.

[0040] The above method can apply to not only ‘rewriting’ but also‘writing’. When a voltage inflicted for writing is increased byincrements, the minimum and maximum values of the writing voltage isvaried to determine the minimum value, the maximum value, and theincremental voltages for each chip according to the test writingcharacteristic as described above. This method can also apply toerasing.

[0041]FIG. 8 shows a semiconductor IC system with a non-volatile memorywhich is mounted together with the IC system on the same board. The ICsystem includes a CPU, a cache CM, a digital signal processor DSP, and anon-volatile NVM that are connected one another via a bus and mounted onthe same board. The CHIP 51 is sealed in one package PKG 5. The NVM inthe semiconductor device is used for, for example, storing a program. Byapplying the present invention to this NVM, the semiconductor device canreduce the program updating time, i.e. the information rewriting time.

[0042]FIG. 9 shows a semiconductor IC system composed of two chips CHIP61 and CHIP 62 (NVM), which are mounted together with the IC system inthe same package PKG 6. The configuration of the IC system is the sameas that shown in FIG. 9. The IC system may includes other chips. Also inthis case, by applying the present invention to the NVM, thesemiconductor device can reduce the program updating time, i.e.information rewriting time. Although not shown in FIG. 9, CHIP 61 andCHIP 62 may be sealed in different packages. Also, it will be understoodapparently that the present invention can apply to the NVM.

[0043] The present invention therefore achieves a short processing time,which is a feature of the block processing, and reduces the variation ofthe threshold voltage, which is a feature of the sector processing.Consequently, the present invention speeds up the whole erasingprocessing.

[0044] The foregoing invention has been described in terms of preferredembodiments. However, those skilled in the art will recognize that manyvariations of such embodiments exist. Such variations are intended to bewithin the scope of the present invention and the appended claims.

What is claimed is:
 1. A method for erasing data from a non-volatilesemiconductor memory device, comprising: selecting a group of word linesamong a plurality of word lines of the memory device; inflicting apredetermined voltage by increments onto each memory cell of each ofselected word lines via the respective word line so as to erase the datatherein by changing a threshold voltage of said each memory cell;checking whether the data in said each memory cell is erased completelyso as to decide whether to inflict next voltage increment on said eachmemory cell, and checking whether all memory cells in each of selectedword lines is erased completely; and blocking unselected word lines andcompletely erased memory cells from being checked.
 2. The method forerasing data from a non-volatile semiconductor memory device accordingto claim 1, further comprising: checking whether said each memory cellis over-erased; rewriting the over-erased memory cells by inflicting apulse voltage by increments onto each of said over-erased memory cells.3. An apparatus for erasing data from a non-volatile semiconductormemory device, comprising: means for selecting a group of word linesamong a plurality of word lines of the memory device; Means forinflicting a predetermined voltage by increments onto each memory cellof each of selected word lines via the respective word line so as toerase the data therein by changing a threshold voltage of said eachmemory cell; Means for checking whether the data in said each memorycell is erased completely so as to decide whether to inflict nextvoltage increment on said each memory cell, and for checking whether allmemory cells in each of selected word lines is erased completely; andmeans for blocking unselected word lines and completely erased memorycells from being checked.
 4. The apparatus according to claim 3, furthercomprising: means for checking whether said each memory cell isover-erased; means for rewriting the over-erased memory cells byinflicting a pulse voltage by increments onto each of said over-erasedmemory cells.